Amphetamine is not something you can take every day and continue getting positive effects from. First of all, the half life is like 12/13 hours for NON extended release amphetamine. That means that a full 24 hours later, 1/4th of your dose is still in your system. As for brand name Adderall XR, the half life will be even longer. Some of the drug will absorb into your feces and take possibly DAYS just to enter your system.
So, knowing that, ask yourself: 'How can it still be in my system if I don't feel any effects from it?' When amphetamine wears off, it's not because your body metabolized or excreted most of it, it's because it has depleted your central nervous system of the resources needed.
In addition to that, being in your system that long constantly telling your CNS to fire up, your brain starts to rewire itself pretty quickly, for a few reasons:
1. Basic Tolerance; Your brain downregulates and upregulates neurotransmitters accordingly to balance itself out as good as it can.
2. Damage to the nervous system - from a combination of oxidative stress (forcing dopamine to stay out in the synapse results in it being metabolized, producing oxygen radicals); overheating; and pathways in your CNS misfiring from the drug, desynchronizing them from normal functioning.
A permanent tolerance develops rapidly to amphetamine without breaks between use. However, this tolerance plateaus at higher doses; this is believed to be because amphetamine stops acting as a dopamine/norepinephrine/5-HT reuptake reverser, and starts behaving as a reuptake inhibitor at around the 75-100mg range. Basically, whatever mechanism amphetamine is using to release dopamine/NE/5-HT into the synapse becomes so saturated with d-amphetamine that reuptake is immediately followed by release again. This is a good way to fry your brain, however.
At lower doses, 20mg of Adderall XR seems to work very well as a treatment for ADD and ADHD, even after tolerance has developed to the 'euphoric' effects. The brain seems to downregulate/upregulate things enough to dull it out, but can't prevent it from still slightly elevating baseline stimulation in the brain.
If you have ADD/ADHD, it should be doing the trick at your dose, although it may not still be getting you high. If you want to get high, you have to take breaks from amphetamine so your brain can replenish; let me know how well that juggling act goes for you though. 95% of the people here told themselves 'I'll just take it once or twice a week, and then get good sleep and eat healthy the rest of the time' and what actually ended up happening is they'd binge use it at high doses (because of that baseline permanent tolerance you get, literally only 90+mg does what you want it to do) and go through cycles of not sleeping a week or two then doing nothing but sleeping and eating for 4 weeks straight, and whenever they try to just keep taking it anyways when theyre already in a psychosis, their brain overheats and they struggle with words and forming sentences and mess up grammar and syntax for 6 months until their brain manages to repair itself.
Some people have never come back from that point though, sometimes because they are homeless without food and miss a critical window when their body needs to be taking in food for their short-term brain damage not to become long-term/permanent brain damage. I don't mean amphetamine psychosis in general, but the overheating and dehydration resulting in brain damage which tends to occur during prolonged amphetamine psychoses, when a user loses sense of time passage and may go 12+ hours without drinking water, going to the bathroom, or even moving into a different position.. it could even be considered catatonic in extreme situations; the biggest problem is a lack of water, instead the brain is telling itself for some reason that not moving is better for it than moving, so it stays in a rigid position for hours, overheating, damaging itself further, leading to further stupor.
